In many transmissions, the gears are preselected by the driver with the gear shift lever, and then the selected gear is engaged by a power-assisted mechanism, such as, a hydraulically or pneumatically operated cylinder. The clutch can be manually operated directly by the driver, or automatically operated by the power-assisted mechanism.
Normally, it takes about 0.6 to 1.5 seconds to change gears. However, if the driver should forget to operate the clutch when shifting, the transmission can be damaged. Damages can also occur if the clutch is not properly operated, namely, if it is applied too quickly or is not completely disengaged when the gear-change occurs.
In order to prevent such damages, it has been previously suggested that the supply of pressure medium required for shifting gears can be shut off by a valve, which is opened only when the clutch is fully disengaged, as shown and disclosed in U.S. Pat. No. 4,354,584. The valve is thereby directly regulated or opened mechanically by the slave cylinder of the clutch.
Another conceivable arrangement has an electrical contact attached to the clutch, which is activated when the clutch is fully disengaged, and which controls an associated solenoid valve.
A disadvantage contained by these known arrangements is that a gear-change can only be initiated with a completely disengaged clutch, although the separation of the clutch linings sometimes occurs as early as two-thirds (2/3) of the clutch pedal travel. This wastes time, as mentioned above, but the transmission can be damaged if the clutch is reengaged too quickly.